1. Field of the Invention
The present invention relates in general to a personal alarm system and specifically to a personal alarm system that includes an interval motion sensor used with a self-contained breathing apparatus such that the motion sensor will set off an audible alarm if motion of the person wearing the breathing apparatus ceases for a predetermined period of time.
2. Description of the Prior Art
There are many instances in which it would be important to have a device that could initiate an audible alarm if motion of a person wearing the device ceases for a predetermined period of time. The intent of this type of device is to enable potential rescuers to locate an individual who may be trapped and who may have lost consciousness during entrapment.
There are many devices in the art which attempt to provide this type of information. In U.S. Pat. No. 5,157,378, issued to Stumberg et al., a motion sensor is associated with pressure and temperatures sensors to provide audible alarms if the pressure in a self-contained breathing apparatus decreases, if the temperature exceeds a certain value or if motion ceases for a predetermined period of time.
U.S. Pat. No. 4,196,429 to Davis has a motion sensor in the hat of a fireman or other worker in a dangerous environment which includes a mechanical sensor, electrical circuitry and alarm system self-contained therein so that the alarm will sound or be otherwise given in an absence of motion for a predetermined period of time thus indicating disablement of the worker or other individual.
There are many problems associated with the prior art devices. Since the device needs to produce an alarm if the movement of the wearer stops for a period of time long enough to assume he cannot move, a human motion detector device is at the core of the needed device. Further, characterization of human motion is difficult at best, but for this product, quantifying the motion is not necessary since a "lack of motion" is what really needs to be detected. It is assumed that human movement is detectable in all three axes simultaneously but detecting a motion in two axes is thought to be sufficient. Further, a sensor for human motion detection needs to be operable with very low mechanical energy input since acceleration associated with human motion can be low amplitude and low frequency. A pendulum principle will function properly because a pendulum typically produces a low frequency oscillatory motion which is sustained by a low energy input. Further, to monitor pendulum motion, opto-electronics are desirable since light-emitting diodes and phototransistors are available in myriads of configurations, are inexpensive, small and do not require mechanical contact. If mechanical contacts were used, a hermetic seal should be provided. An electronic circuit for such device having a phototransistor signal as an input should sense motion throughout all 360.degree. in one plane or about one axis. The resolution of the detection depends on the mechanics of the device.